It is known that the mechanical performance of structural elements varies as Ex/d, the coefficient x depending on the mode of external stressing (for example in tension or in bending) or on the geometry of the elements (plates, bars). This illustrates the benefit of having materials exhibiting both a high elastic modulus and a low density.
This requirement applies most particularly in the automotive industry where vehicle lightening and safety are constant preoccupations. Thus the aim is to increase the elastic modulus and reduce the weight of steel parts by incorporating ceramic particles of various types, such as carbides, nitrides, oxides or borides. The reason for this is that such materials have a markedly higher elastic modulus, ranging from about 250 to 550 GPa, than that of base steels, which is around 210 GPa, into which they are incorporated. In this way, hardening is achieved by load transfer between the matrix and the ceramic particles under the influence of a stress. This hardening is increased further due to the matrix grain size refinement by the ceramic particles. To manufacture these materials comprising ceramic particles distributed uniformly in a steel matrix, processes are known that are based on powder metallurgy: firstly, ceramic powders of controlled geometry are produced, these being blended with steel powders, thereby corresponding, for the steel, to an extrinsic addition of ceramic particles. The powder blend is compacted in a mold and then heated to a temperature such that this blend undergoes sintering. In a variant of the process, metal powders are blended so as to form the ceramic particles during the sintering phase. Despite mechanical properties improved over steels not containing a dispersion of ceramic particles, this type of process suffers from several limitations:
it requires careful smelting and processing conditions in order not to cause a reaction with the atmosphere, taking into account the high specific surface area of metal powders;
even after the compacting and sintering operations, residual pores likely to act as initiation sites during cyclic stressing may possibly remain;
the chemical composition of the matrix/particle interfaces, and therefore their cohesion, is difficult to control given the surface contamination of the powders before sintering (presence of oxides and carbon);
when the particles are added in large quantity, or when certain large particles are present, the elongation properties decrease;
this type of process is suitable for low-volume production but cannot meet the requirements of mass production in the automotive industry; and
the manufacturing costs associated with this type of manufacturing process are high.
In the case of light alloys, manufacturing processes are also known that are based on the extrinsic addition of ceramic powders into the liquid metal. Here again, these processes suffer from most of the abovementioned drawbacks. More particularly, the difficulty of homogeneously dispersing the particles may be mentioned, such particles having a tendency to agglomerate or to settle in or float on the liquid metal.
Among the known ceramics that could be used to increase the properties of steel is in particular titanium diboride TiB2, which has the following intrinsic characteristics:
Elastic modulus: 565 GPa;
Relative density: 4.52.
However, since the manufacturing processes rely on extrinsic additions of TiB2 particles, they suffer the abovementioned drawbacks.